Field of the Invention
This invention is related to a process for the beneficiation of phosphate ores. More particularly, this invention relates to a process for the beneficiation of phosphate ores having a gangue containing dolomite and/or calcite as well as silica.
Phosphates obtained from phosphate ores are used for the production of phosphoric acid. To be economically marketable, the phosphates must have a low impurities content. Among the most detrimental impurities found in phosphates are iron, aluminum, and magnesia. Phosphate ores having a magnesia-rich dolomitic gangue and which also contain calcite are found in Florida. Because of the high magnesia content, such phosphate ores have been considered uneconomical to process.
According to the processes known in the prior art and used in Florida for the treatment of phosphates with a siliceous gangue, the ore is first washed and classified into four fractions which are processed separately. The fine particles, i.e., those having a diameter of less than about 104 microns (i.e., those which pass through a 150 mesh Tyler sieve series), and which consists mainly of clays, are considered to be waste. The other three fractions have particle diameters ranging from about 104 to 417 microns (i.e., those which pass through a 35 mesh Tyler sieve series and are retained on a 150 mesh Tyler sieve series), from about 417 to 1000 microns (i.e., those which pass through a 16 mesh Tyler sieve series and are retained on a 35 mesh Tyler sieve series) and greater than about 1000 microns (i.e., those which are retained on a 16 mesh Tyler sieve series). Among these three fractions, the fraction having a particle diameter exceeding about 1000 microns is sufficiently rich so as not to require any treatment.
The fractions of particles having a diameter ranging from about 104 to 417 microns and from about 417 to 1000 microns are subjected to a flotation process. The flotation process is normally performed in two stages. The first stage comprises an anionic flotation of phosphates either in flotation cells for the fine particles (104-417 microns, i.e., 150-35 mesh), or on belts for the coarse particles (417-1000 microns, i.e., 35-16 mesh). In the course of the flotation, the larger part of the silica is taken along with the residue. To further improve the quality of the resulting phosphate product, a second stage flotation to eliminate the rest of the silica is necessary. This is accomplished for both the fine and coarse particles by washing, and conditioning the particles before subjecting the two fractions to a cationic flotation, which is designed to eliminate the residual silica.
The above-described process has been in use for many years. However, this process cannot be used for the treatment of phosphate deposits having a carbonate gangue containing calcite and dolomite in addition to the usual silicates. Consequently, the above-mentionned process cannot be used for beneficiating the carbonate rich layers of the phosphate deposits found in Florida. This is due to the fact that carbonates have the same behavior as phosphate particles under usual industrial flottation conditions. As a result, the carbonate particles cannot be removed from the phosphate particles. Apparently, the affinity of carbonates and phosphates for fatty acids in anionic flottation processes is about the same, thus preventing separation thereof.
A large number of studies have been directed to the removal of carbonates from phosphate ore having a carbonate-containing gangue. For example, British Patent No. 859,155, which corresponds to U.S. Pat. No. 3,113,838, discloses a process applicable to phosphate-rich rock containing calcium carbonate. The process comprises grinding the rock to 200 microns to liberate the phases, floating the carbonates at a pH of 6 to 7 with fatty acids while depressing the flotation of the phosphates by adding soluble phosphate salts to the solution obtained. However, the process disclosed in this reference is not applicable to ores which have both carbonate and silicate gangue.
Another known process applicable to essentially calcitic ore is described in U.S. Pat. No. 3,259,242. In this process, the cationic flottation of the phosphates at a pH of 7 is disclosed. Flotation is followed by washing with SO.sub.2 or H.sub.2 SO.sub.4. However, this process cannot be applied to ores having a siliceous gangue since it is well known in the art that the silicates are equally floatable with amines used as cationic flotation agent. Furthermore, the use of sulfuric acid or sulfur dioxide incurs an additional expense, thus increasing the cost of the process.
Another cationic flotation method for phosphate is described in U.S. Pat. No. 4,144,969 for ore with a carbonate and silica (less than about 20% by weight) containing gangue. Apatite is floated with a cationic reagent at a pH of from about 5 to 6.5. Carbonates are eliminated in the tailings of the flotation. The pH is maintained with different acids such as acetic, phosphoric, hydrochloric, nitric and hydrofluoric acids.
Other processes have been developed wherein the elimination of phosphates by flotation is described. For example, as described in Mining Engineering, January 1978, at pages 37 to 40, a process utilizing the elimination of phosphates by means of H.sub.2 SiF.sub.6 is described. Similarly, U.S. Pat. Nos. 3,462,016, 3,462,017, and 3,807,556 disclose processes for treating ores having a carbonate- and silica-containing gangue by using two anionic flotation steps. In particular, the technique which has been described in U.S. Pat. Nos. 3,462,016 and 3,462,017, and which seemed to be the most promising one, does not lead to concentrates that can be considered marketable.
That technique consists in a first stage designed to produce a fine grind (d.sub.80 =200 mesh or 74 microns), followed by a second stage for bringing about an anionic flotation by means of an anionic collector used by itself (without any fuel oil), followed by a third stage intended to take up the concentrate which is still impregnated with the collector, with a small additional amount, if the need for that is felt, and to float the carbonates at a pH that differs from the one used during the first stage.
When the technique described in the patents specified above is applied to the magnesium phosphates of Florida, it does not make it possible to achieve marketable concentrations, even when the technique is modified thoroughly. As a matter of fact, it is not possible to obtain phosphates the grade of which amount to more than 60% of BPL and to less than 1% of magnesium (Mg O/BPL less than 60), with a yield exceeding 80% of the material subjected to the flotation process (see also Example 6).
None of the techniques described above produces satisfactory results when they are applied to phosphate ores from Florida; the average composition of which lies within the following ranges:
______________________________________ Non-soluble (essentially silica) 45% to 65% Ca.sub.3 (PO.sub.4).sub.2 10 to 20% MgO 3 to 6% Al.sub.2 O.sub.3 2 to 5% Fe.sub.2 O.sub.3 0.5 to 2% ______________________________________
The classification in different groups which makes the liberation of particles of carbonate and phosphate possible is insufficient when the phosphate particles are covered with a thin coating of carbonate or a clay which forms the cement that holds the phosphate grains together. If the surface phosphate layer is contaminated with carbonate traces, the flotation selectivity is changed due to the fact that phosphates behave exactly like carbonates, thus requiring the presence of phosphate-eliminating agents which increases the cost of the process. In addition, the separation of phosphate and silica cannot be carried out in a single flotation step. An additional cationic flotation step is required in accordance with a technique presently used in the treatment of phosphate ores from Florida.
From the above, it is apparent that there exists a need for a process for treating phosphate ores containing a magnesia-rich carbonate-containing gangue (dolomite) and a silica-containing gangue.
Accordingly, it is an object of the present invention to provide a process for the beneficiation of phosphate ores having a gangue containing dolomite and/or calcite as well as silica.
It is a further object of the present invention to provide a process for the beneficiation of phosphate ores whereby the carbonates contained in the gangue are advantageously liberated and removed.
It is yet another object of the present invention to provide a phosphate ore beneficiation process whereby the magnesia content of the phosphate product is reduced advantageously.
It is another object of the present invention to provide a beneficiation process for phosphate ores, which process is inexpensive and does not require the use of expensive reagents.